Quantitative thymidine-radiography is used to establish the time of origin of neurons throughout the central nervous system of the rat. These data are correlated with embryonic studies in an attempt to infer the site of origin of the dated neurons and their routes of migration. The differentiation of neurons is studied with an X-ray procedure and the Golgi technique. Present efforts concern the development of the medulla, the pons and the midbrain. In the behavioral aspect of the project, the cerebellum, the hippocampus, or the olfactory bulb is exposed to focal X-ray to prevent the formation of postnatally acquired granule cells. The effects of this specific retardation of regional brain development are assessed with a variety of behavioral techniques. These tests include spatial, tactile and visual discrimination learning in hippocampal animals; detection of brief and mobile cues in the cerebellar animals; and olfactory discrimination learning and feeding patterns in the olfactory animals.